1. Field
The present disclosure generally relates to systems and methods of actuation, and, in particular, relates to the actuators utilizing memory alloy wire.
2. Description of the Related Art
Providing secure storage frequently requires a container with a lid that is released only after certain requirements are met, such as verification that the individual accessing the container is authorized to do so. Systems of this type use an actuator of some type to release a latch that otherwise retains the lid in the closed position. Common actuators include solenoids and motors, both of which may be relatively large compared to the usable volume of the container, which decreases the volumetric efficiency of the container.
Actuators that utilize memory alloy wire can provide sufficient power and stroke to release the latch of a secure container while occupying less volume than a solenoid or motor. Memory alloy wire, also known as “muscle wire,” is made from one of a number of alloys that contract in length when heated and can be stretched back to their original length when cooled back to room temperature. Example alloys include nickel-titanium alloys that were first developed by the US Naval Ordnance Laboratory and commercialized under the trade name Nitinol (taken from the words Nickel Titanium Naval Ordnance Laboratories). The memory alloy wire is commonly heated by passing an electric current through the wire, creating heat within the wire due to the internal resistance of the wire.
When used as an actuator, a length of memory alloy wire typically has a terminal attached to each end of the wire. Commonly available memory alloy wire actuators have terminals that attach to posts on printed circuit board assemblies (PCBAs) and serve as the electrical contact for the current that heats the memory alloy wire as well as the mechanical attachment. The PCBA is then mounted to the same structure to which the other elements of the actuator are attached, adding another assembly tolerance to the system. One drawback of current memory alloy wire actuators is that shape memory strain is typically limited to 5%, which translates to a maximum stroke of 0.100 inches for a 2 inch actuator. This stroke can easily be consumed by the sum of multiple assembly tolerances, leaving little usable stroke for the actual release function. An additional drawback is that the memory alloy is sensitive to fatigue at points of stress concentration due to bends in the wire at the point of electrical termination.
U.S. Pat. No. 6,116,461, Method and Apparatus for the Dispensing of Drugs, Broadfield et al., discloses an Automated Dispensing Machine (ADM) that utilizes a memory alloy wire actuator. While this system was a significant advance in the dispensing of medications, the memory wire is directly and rigidly attached to the PCBA as described above. As such, the memory alloy wire actuator does not reach its full potential.